1. Field of the Invention
The present invention relates to a recording apparatus employing a magneto-optical effect for recording an information signal onto a magneto-optical disc and, more particularly, to a magneto-optical recording apparatus having a modulated magnetic field.
2. Description of the Prior Art
In a magneto-optical disc recording/reproducing apparatus having a modulated magnetic field, a laser beam irradiates a region of the recording surface of a magneto-optical disc, and a magnetic field modulated by recording data is applied to such region of the magneto-optical disc for recording the data thereon. According to such a magnetic field modulation system, a so-called overwrite can be executed in which new data are written without erasing earlier written data.
A proposed magneto-optical disc recording/reproducing apparatus of the magnetic field modulating type uses continuous irradiation of the magneto-optical disc by a laser beam. However, in such a magneto-optical disc recording/reproducing apparatus of the magnetic field modulating type using continuous irradiation, it is difficult to produce a magnetization pattern whose orientation is suddenly reversed between adjacent recorded magnetic domains.
More specifically, in the magneto-optical disc, a perpendicular magnetizing film is heated to a Curie temperature Tc or higher during the irradiating time of the laser beam. When an external magnetic field is applied and the perpendicular magnetizing film is then cooled to a Curie temperature Tc or less, the orientation of the magnetic field of the perpendicular magnetizing film is set. Therefore, in order to produce a magnetization pattern whose orientation is suddenly reversed between adjacent magnetic domains, the external magnetic field needs to be maintained at a saturation level or higher until the perpendicular magnetizing film can be cooled to the Curie temperature Tc or less.
In the magneto-optical disc recording/reproducing apparatus of the magnetic field modulating type employing continuous irradiation, the magneto-optical disc is rotated until the region in which a magnetic domain is produced is deviated from a beam spot, and thereafter, such region is cooled. Therefore, the external magnetic field corresponding to the region between input coded data is applied before the perpendicular magnetizing film of the region in which the magnetic domain is produced is cooled to the Curie temperature Tc or less and the orientation of the magnetic field of the perpendicular magnetizing film is set. Accordingly, if the recording data have been inverted, a region in which the magnetization orientation is undetermined is apt to occur between adjacent magnetic domains.
In particular, in the magneto-optical disc recording/reproducing apparatus of the magnetic field modulating type using continuous irradiation, as represented in FIG. 1A, a predetermined laser beam is irradiated onto the magneto-optical disc. As further shown in FIG. 1B, a magnetic field modulated by recording data is applied to the magneto-optical disc. On FIG. 1B, each of the dotted lines H indicates a magnetic saturation level.
When the laser beam is continuously irradiated as described above, the external magnetic field at change or transition points in the input data is applied before the perpendicular magnetizing film can cool to the Curie temperature Tc or less. Therefore, as shown in FIG. 1C, a region A10 whose orientation is undetermined occurs between adjacent recorded magnetic domains.
Therefore, it has been proposed to proved a magneto-optical disc recording/reproducing apparatus of the magnetic field modulating type with a pulsed irradiation, and in which a laser beam is intermittently irradiated onto a magneto-optical disc in response to predetermined reference clocks and a magnetic field modulated by recording data is simultaneously applied with the laser beam irradiation (International Publication No. WO89/12889). In such a magneto-optical disc recording/reproducing apparatus of the magnetic field modulating and pulsed irradiation type, since the laser beam is intermittently irradiated, the external magnetic field can be set to a saturation level or higher until the perpendicular magnetizing film can cool to the Curie temperature Tc or lower and the orientation of the magnetization of the perpendicular magnetizing film is set. Thus, a magnetization pattern whose orientation is suddenly reversed between adjacent recorded magnetic domains can be produced.
More specifically, as shown on FIG. 2A, in the magneto-optical disc recording/reproducing apparatus of the magnetic field modulating and pulsed irradiating type, the laser beam is intermittently energized, while, as shown in FIG. 2B, the magnetic field modulated by recording data is applied to the magneto-optical disc.
In such case, the orientation of the magnetization of the perpendicular magnetizing film is determined by the external magnetic field which is applied for a period of time T.sub.A concluding when the temperature of the perpendicular magnetizing film is cooled to the Curie temperature Tc or lower after the laser beam has been energized or turned on for irradiating the magneto-optical disc. Once the temperature of the perpendicular magnetizing film has been cooled to the Curie temperature Tc or lower, the orientation of the magnetization of the perpendicular magnetizing film of the magneto-optical disc does not change.
As shown in FIG. 2B, in the magneto-optical disc recording/reproducing apparatus of the magnetic field modulating and pulsed irradiating type, an external magnetic field of a saturation level H or higher is applied during each time T.sub.A. Therefore, as shown in FIG. 2C, a magnetization pattern whose orientation is suddenly reversed between adjacent magnetic domains can be formed.
On the other hand, in the above-described conventional magneto-optical disc recording/reproducing apparatus of the magnetic field modulating and pulsed irradiating type, the minimum number of drive pulses needed to form recorded magnetic domains of one channel bit is determined by the modulation system. More particularly, the minimum number of pulses which are needed to form the recorded magnetic domains of one channel bit is determined by a ratio N of a minimum length Tmin between transitions of the modulation system to a window margin or clock period Tw.
If, for example, a NRZI (Non Return to Zero Inverted) modulation system is utilized in which the minimum length Tmin between transitions is T and the window margin Tw is also T (T denotes a channel byte interval), the minimum number of drive pulses which are needed to form the pits of one channel bit is 1. However, if a 2-7 modulation system is utilized in which, for example, the minimum length Tmin between transitions is 1.5T and the window margin Tw is 0.5T, the minimum number of drive pulses which are needed to form the recorded magnetic domains of one channel bit is 3.
Therefore, even if the recording densities are the same, when the modulation system is changed, then the interval of necessary drive pulses changes. More specifically, if 2-7 modulation is used as the modulation system, then the interval of drive pulses needs to be 1/2 of the interval of drive pulses needed in the case where NRZI modulation is the modulation system.
In the above described magneto-optical disc recording/reproducing apparatus of the magnetic field modulating and pulsed irradiating type, if the interval of drive pulses is reduced by at least a predetermined interval which is determined by the thermal response speed of the magneto-optical disc, the laser beam is energized by the next drive pulse before the perpendicular magnetizing film can cool to the Curie temperature Tc or lower and hence before orientation of the magnetization is set. Accordingly, in this conventional magneto-optical disc recording/reproducing apparatus of the magnetic field modulating are pulsed irradiating type, if the modulation system is changed from NRZI modulation to 2-7 modulation and the interval of drive pulses is reduced, a problem exists in that a magnetization pattern whose orientation is suddenly reversed between adjacent magnetic domains cannot be formed.
Further, another system has been considered in which the thermal response speed of the magneto-optical disc is improved to thereby make it possible to produce a magnetization pattern whose orientation is suddenly reversed between adjacent pits, even in the case where 2-7 modulation is used. However, limitations exist in respect to the possible improvement of the thermal response speed of the magneto-optical disc.